Asteroseismic tests for models of magnetically active close binaries with orbital period modulation

Abstract

Magnetic activity and orbital period modulation in close binaries are considered in order to identify possible aster- oseismic tests for models that have been proposed to understand their relationship. These models are based on the variation of the gravitational quadrupole moment of the active star, as a consequence of the modification of its internal rotation and magnetic field along its activity cycle (Applegate 1992; Lanza et al. 1998a). An analysis based on the solar analogy shows that surface magnetic activity may produce shifts, splitting and broadening of the p-mode peaks in the oscillation spectrum of a star. Such eects make the direct detection of the internal structural changes predicted by the models very dicult. However, a time variation of the internal rotation can be detectable because the p-mode rotational splittings are proportional to the azimuthal quantum number m, whereas structural and surface eects controlled by the magnetic fields arejmj dependent. Therefore, we propose a method that correlates the variation of the a1 splitting coecients, that sample the internal rotation, with the orbital period variation in order to test the predictions of the models. The RS CVn short-period systems, such as V 471 Tau, RT And or CG Cyg, are the most suitable objects for such a test because surface magnetic eects are likely to be less prominent than in classical or long-period RS CVn binaries.